A current mirror is a current-controlled current source wherein the current output is proportional to the current input. A basic MOS transistor current mirror includes two MOS transistors having the same gate-to-source voltage. With both transistors operating in the saturation region, and the gate and drain of the first transistor coupled together, the current through the second transistor will be approximately proportional to the current through the first transistor independent of the drain-to-source voltage on each transistor. However, when the second transistor is operating in the triode region (low drain-to-source voltage), the basic MOS transistor current mirror breaks down because the current is no longer independent of drain-to-source voltage.